A longstanding problem in telephone communication systems has been that of providing loop operating current to a subscriber telephone line circuit in an efficient manner while assuring proper operation of the loop with minimum drain on the central office battery. In a digital telephone subscriber loop system, the problem becomes one of minimizing the power dissipated at a signal processing terminal that is remote from a central office. An electronic line feed circuit at the remote terminal must be able to supply ample current for normal operation while a subscriber station is off hook. The amount required depends upon the length of the subscriber line circuit. A current profiling feedback circuit is often included in a line feed circuit to determine when and how the available current for the line must change to accommodate a particular line length. In a strategy which tries to keep power dissipation at the remote terminal constant, independently of loop length, the current profile reduces to a piece-wise linear function. Circuits known in the art for producing piece-wise linear current functions are usually relatively complex and require substantial silicon area if in integrated circuit form or substantial labor cost if in discrete component form.
One example of a technique for producing a piece-wise linear current characteristic is found in a P. C. Davis et al. paper "A Hybrid Integrated Trunk and Subscriber Line Interface", 1982 IEEE International Solid-State Circuits Conference, pp. 204-205. There an operational amplifier is provided for cooperation with multiple feedback circuits for providing several different load-line options.
Another example is found in Introduction to Operational Amplifier Theory And Applications by J. V. Wait et al., McGraw-Hill Books, 1975, at pp. 171-187. Wait et al. describe several ways to implement a function so that it varies linearly between breakpoints. All of the implementations described are built up around operational amplifiers; and most are voltage, responsive circuits.